1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus on which the zoom lens can be used. More specifically, the present invention relates to a zoom lens useful as a photographic lens used in an image pickup apparatus, such as a digital still camera, a video camera, a surveillance camera, a television (TV) camera, and a film camera, and an optical apparatus having the zoom lens.
2. Description of the Related Art
Current market forces demand image pickup apparatuses with reduced size and increased functionality. To that end, newly designed photographic optical systems used in an optical apparatus, such as a video camera, a surveillance camera, or a digital still camera, are desired to be small and highly versatile. In particular, a small-sized zoom lens having a wide angle of view, a high zoom ratio, and a high optical performance for the entire zooming range is highly desirable.
A conventional four-unit zoom lens, which includes, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power, has already been proposed. Examples of the conventional four-unit zoom lens are described in Japanese Patent Application Laid-Open No. 11-305129 and Japanese Patent Application Laid-Open No. 2006-171431. Both of these patent applications discuss a rear-focus type four-unit zoom lens that executes variable magnification by moving the second lens unit and corrects the variation on the image plane that may occur during variable magnification and executes focusing by using the fourth lens unit.
A rear-focus type zoom lens is more useful than a zoom lens that executes focusing by moving the first lens unit due to the following characteristics. In a rear-focus type zoom lens, the effective diameter of the first lens unit is smaller than the diameter in the zoom lens that executes focusing by moving the first lens unit. Accordingly, it becomes easier to reduce the total size of the lens system. In addition, in the rear-focus type zoom lens like this, a small-size lens unit is moved to execute variable magnification and focusing. Accordingly, a smaller driving force is required to drive the lens unit. Therefore, quicker focusing can be achieved if a rear-focus type zoom lens is used.
However, if the rear-focus type zoom lens described above is used, variation of aberration that occurs during focusing may increase. Therefore, it may become difficult to achieve a high optical performance for the entire area of focusing (i.e., the object distance) from an infinitely-distant object to a short-distance object.
In order to achieve a wide angle of view on a small-size rear-focus type four-unit zoom lens, it is significant to appropriately arrange and set the lens configuration of the magnification-varying second lens unit having a negative refractive power and the third lens unit having a positive refractive power. Unless the shape of each lens that constitutes the second lens unit or the lens configuration of the third lens unit is appropriately set, it may become difficult to achieve an optical system (zoom lens) whose total size is small and having a predetermined high zoom ratio. Accordingly, in this case, variation of various aberrations that may occur during zooming may increase. Therefore, it becomes difficult to achieve a high optical performance.